(a) Technical Field
The present invention relates to a mixture of a nickel-based catalyst and a metal oxide as a catalyst for preparing a syngas by reforming methane, more particularly to a metal catalyst including magnesia, nickel, vanadium, tungsten, iron, molybdenum and chromium supported on an alumina support, which exhibits inhibited carbon deposition and improved catalytic activity.
(b) Background Art
The technology of liquefying natural gas has been drawing attentions since it can reduce emissions of carbon dioxide, which is the main cause of global warming, produced when natural gas obtained as byproduct in oil fields is burnt. The gas-to-liquids technology is also the key protect of major petroleum companies because of technical and politic advantages such as utilization of medium-to-small-sized gas resources, coping with depletion of petroleum resources, or the like. Reforming, which is an indispensable technique in liquefying natural gas, is preoccupied by foreign conglomerates. Since the natural gas liquefaction requires more than 60% of investment cost initially, studies are intensively under way to reduce the investment cost.
In the natural gas liquefaction technology, production of hydrogen and carbon monoxide through reforming is a very important process and researches are actively under way on catalysts used in this process. The reforming techniques for preparing a syngas from methane include steam reforming, carbon dioxide reforming, partial oxidation reforming reaction, or the like.
Steam reforming of methane follows Reaction Formula 1 and a nickel-based catalyst is used in general.CH4+H2O=CO+3H2 HΔ298=+206 kJ/mol  [Reaction Formula 1]
During the steam reforming reaction, the catalyst becomes inactivated due to sintering and carbon deposition. In order to prevent this problem, a large amount of steam is supplied. Further, addition of a cocatalyst or modification of a catalyst preparation method is studied to prevent carbon deposition.
The carbon dioxide reforming reaction of methane follows Reaction Formula 2 and a nickel-based catalyst is used in general.CH4+CO2=2CO+2H2 HΔ298=+247.3 kJ/mol  [Reaction Formula 2]
The carbon dioxide reforming reaction is significant in that it consumes methane and carbon dioxide which are major contributors to global warming. The reaction is strongly endothermic and reaction activity increases with temperature. Further, this reaction is characterized in that carbon formation is thermodynamically favored since the reaction gas has a high carbon-to-hydrogen ratio. In order to prevent catalyst deactivation due to the carbon deposition, a cocatalyst or a noble metal-supported catalyst is used. Although the noble metal-based catalyst exhibits good activity, it is too expensive to be industrially used.
The steam carbon dioxide reforming reaction of methane follows Reaction Formula 3.3CH4+CO2+2H2O=4CO+8H2  [Reaction Formula 3]
This reforming reaction is advantageous in that the carbon deposition of the steam reforming and carbon dioxide reforming reactions can be solved and an H2/CO ratio adequate for the Fischer-Tropsch (F-T) synthesis can be achieved.
Regarding catalysts for the methane reforming reaction, Applied Catalysis A: General, 340 (2008), 183-190 discloses a Ni/MgO/Al2O3 catalyst prepared by the incipient method on an alumina support for Steam and Carbon dioxide reforming of methane. More specifically, it describes preparation of the Ni/MgO/Al2O3 catalyst and improvement of resistance to carbon deposition using magnesia as a promoter. In the present invention, a metal oxide is added in addition to the Ni/MgO/Al2O3 catalyst to maximize resistance to carbon deposition by gasifying carbon.
Chemical Engineering Journal, 168 (2011), 775-783 discloses a catalyst having a core/shell structure for steam reforming of methane. In contrast, in the present invention, a catalyst is prepared by an impregnation method and it is used for Steam and Carbon dioxide reforming of methane.
Korean Patent Application No. 10-2005-0099407 (Oct. 20, 2005), which relates to a nickel-based catalyst for producing a syngas by tri-reforming reaction of methane and a tri-reforming reaction of methane using the catalyst, discloses a catalyst wherein a nickel (Ni) metal is supported on a zirconia support doped with a metal selected from an yttrium (Y) metal, a lanthanide element and an alkaline earth metal element. However, the catalyst using the yttrium (Y) metal, the lanthanide element and the alkaline earth metal element as promoter is limited in the life of the catalyst because it lacks resistance to carbon deposition.
Korean Patent Application No. 10-2010-0011687 (Jul. 25, 2008), which relates to a method for preparing a catalyst for carbon dioxide reforming of methane and a reforming method using same, discloses a method of preparing a catalyst by dipping a mesoporous silica molecular sieve carrier in an aqueous solution of nickel nitrate and then drying and sintering and describes that the conversion rate of methane and carbon dioxide and the yield of a mixed gas can be stably maintained since deactivation due to carbon deposition is prevented. However, because it is not easy to achieve resistance to carbon deposition at a relatively high pressure of 20 bar, it is not applicable to a process associated with Fischer-Tropsch (F-T) synthesis. In addition, the catalyst is applicable only to carbon dioxide reforming of methane and does not have resistance to carbon deposition when used for methane reforming combining steam reforming and carbon dioxide reforming as in the present invention.
To conclude, although the steam carbon dioxide reforming (SCR) of methane combining steam reforming and dry reforming is attractive in that it utilizes CO2, a commercial-scale process has not been developed yet because of catalyst deactivation due to carbon deposition.
Throughout the specification, a number of publications and patent documents are referred to and cited. The disclosure of the cited publications and patent documents is incorporated herein by reference in its entirety to more clearly describe the state of the related art and the present invention.